As someone who has been working with raster images (pixels) since the 90s, I just roll my eyes when the term "Mega Pixel" is used. It's really a nonsense term, so there is no reason to discuss it. What you have to know is pixels, and it starts with knowing how big a pixel is and how to measure it.
There are 72 pixels in an inch. That's a regular, linear inch. Not a square inch. Look at your screen right now. If you're a man with an average-sized hand, stretch it out, thumb-to-pinky-finger. That's about 10 inches. Since there are 72 pixels in an inch, that's 720 pixels.
Now look at your computer screen. If you're on a nice iMac like I am, it's a little over two hand-spans wide. In rough numbers, it's about 2000 pixels wide. If you want to share a photo with me that I can see in an email, or even on a web page, there is no reason to make the image wider than my entire screen. In fact, much of my width is used up by the frame around it, and the boxes that hold my browser and my email program.
The best place to start is with the settings of your camera. Yes, you paid top dollar for gigantic "Mega Pixel" images, but they're being saved as 200,000 pixels wide, or something ridiculous like that. Actually, I don't know. It's just "Mega", I guess. And that means that, in addition to being way too big to share, they're gigantic files that could never be emailed. So, uh, set your camera for its lowest setting.
If you have a software program, like Photoshop, that will tell you how wide your images are, I would suggest no more than 1600 pixels @ 72 pixels per inch. If you click on the picture of Macintosh, the good little wiener dog, below, you will see it display on your screen at that size. This not only displays beautifully on a browser, it is easily sent in email.
|Click on this image to see it at full size, 1600 pixels wide|